(1) Field of the Invention
The present invention relates to digital audio technology for converting an analog signal to a digital signal and recording and reproducing the digital signal, and particularly relates to bit extension technology for recreating amplitude information lost due to quantization during digitalization of an analog signal.
(2) Description of the Related Art
Digital audio technology for digitalizing an analog music signal and recoding and reproducing the digital signal is the current is predominant audio technology because digital audio technology provides extremely high signal recreatability, digital signal processing facilitates LSI-implementation, and implementation through small-sized and power-saving configurations is possible at an inexpensive price.
However, since quantization and sampling processes are performed with predetermined values for the quantization bit number and the sampling frequency in the conversion of analog signals to digital signals, it is not possible to recreate minute signals that are below the resolution capability. Furthermore, it is likewise not possible to accurately quantize a low-frequency signal, even when it is a signal having large amplitude, because the change in amplitude is small in terms of sampling interval. As such, recreatability of minute signals deteriorates. Furthermore, there are disadvantages in terms of sound quality, such as the gradation of low frequency signals becoming unclear.
Consequently, there has been conventionally proposed a bit extension technique of performing prediction and filtering processes on an audio signal having a quantization error, converting such audio signal into a signal having a larger quantization bit number than the original digital audio signal, and thus reducing quantization noise.
For example, a technique of extending the bit count of an input signal by N bits by multiplying the value representing the input signal by 2N, then applying a low-pass filter to the bit-extended signal is commonly performed. With this, the values of the bit-extended input signal are smoothly interpolated. However, there is the problem that the high-frequency component of a signal is lost due to low-pass computation.
FIG. 13 is a diagram showing an example of a conventional quantization distortion reducing apparatus.
Hereinafter, an operation of the conventional quantization distortion reducing apparatus shall be described with reference to FIG. 13.
In FIG. 13, digital low-pass filters 103 to 105 extract low-frequency components equal to or lower than a predetermined cut-off frequency from an input digital signal D0 while extending the bit length in the lowest-bit direction. Meanwhile, a counter 102 counts, for each point of the input digital signal D0, the number of points in which the same value continues before and after such point. Subsequently, based on the counting result from the counter 102, a selection unit 106 (i) selects and outputs, for a section having a count value lower than a predetermined value, a digital signal D1 which has been time-adjusted and has had its bit length extended in the lowest-bit direction by a delayer 108 and a bit extension circuit 109, respectively, out of the input digital signal D0, and (ii) selects and outputs, for a section having a count value equal to or greater than the predetermined value, a signal that is selectively passed through a filter having a lower cut-off frequency among the digital low-pass filters 103 to 105 as the count value is larger, thereby extending the quantization bit number. With this, it is possible to select a filter having a low cut-off frequency when there are many points where the same value continues before and after, that is, when the frequency is low, and select a filter having a high cut-off frequency when there are few points where the same value continues before and after, that is, when the frequency is high (see Patent Reference 1).